This application is a continuation-in-part of my copending U.S. patent application Ser. No. 921,165 filed Oct. 20, 1986, and now abandoned.
This invention relates to nonwoven fibrous materials (hereafter "nonwovens" for brevity) and to a process for bonding fibers in nonwovens with a crosslinked carboxylated acrylate latex. The term "nonwoven fibrous material" is used herein to define a consolidated mass of fibers laid down by mechanical, chemical, pneumatic, electrical or vacuum means, or otherwise deposited on either a flat or three dimensional surface.
Such fibers are commonly used to produce both (i) laminar goods in a weight ranging from about 0.2 oz/yd.sup.2 to about 100 oz/yd.sup.2 and having a thickness of from about 5 mils to 10 inches or more, referred to as a web, mat or sheet (all of which are referred to herein as "web"); and (ii) non-laminar goods of arbitrary shape formed by molding a mass of fibers either just before an acrylate latex is applied to it, or soon after the latex has been applied to it, but before the latex is cured. In either case, the end result desired is a resilient, relatively bulky, non-woven fibrous mass of controlled thickness and shape, adapted for use in a specific application.
Products made from such nonwovens, comprise randomly arrayed fibers or filaments, having a carded fiber structure, or comprising fibrous mats in which the fibers are distributed haphazardly, known in the art as a "random array". Loosely assembled fibers forming a relatively thick fabric from about 1 cm to about 25 cm thick, may be used as insulation and protective padding: while a relatively thin fabric from about 1 mm to about 1 cm thick may be used for textile products such as bedsheets, aprons, disposable gowns, curtain and drapery stock, and the like, the physical properties of the fibers and the amount of latex cured on the fibers, determining the resilience and "hand" of the cured nonwoven.
In prior art, such as in U.S. Pat. No. 4,059,665 to Kelley, nonwovens are coated with a latex which is then cured on the fibers (hereafter referred to as "treated nonwoven"), to provide essential qualities such as resilience, solvent resistance, "softness" and crush or compression resistance also referred to as `shape retention` or `memory`. The nonwoven of this invention not only provides such qualities but also provides improved lack of adhesion or `lack of blocking` between pressed-together surfaces of nonwovens under high pressure and elevated temperature.
`Blocking` describes the adhesive-like bonding of one treated surface to another when the surfaces are pressed into contact. Thereafter separating the surfaces results in a sufficiently large disruption of the interactive bonding between the surfaces as to damage the pulled-apart surfaces. Such blocking is usually lacking in nonwovens treated with a latex which yields a polymer having a glass transition temperature (T.sub.g) above about 10.degree. C., but is often a characteristic of nonwovens treated with a latex which results in a polymer having a T.sub.g lower than about -10.degree. C. Blocking is particularly noticeable with some carboxylated acrylate polymers having a T.sub.g lower than about -20.degree. C., below which many such latexes are classified as pressure-sensitive adhesives (PSAs). In sharp contrast, treated nonwovens of this invention are non-blocking despite having a T.sub.g in the range from about -20.degree. C. to -60.degree. C. Such non-blocking is evident in a tightly rolled nonwoven which does not adhere to itself even above 100.degree. C. (212.degree. F.).
Of course, blocking is of little interest at about the T.sub.g of the fibers, or the temperature at which the cured film is degraded. But blocking is a great concern at such ambient temperatures and high compressive forces as prevail during shipment of treated nonwoven goods, or, those at which the treated goods are stored. Persistent blocking sometimes requires interleaving the goods with "non-stick" sheets. One commercially available carboxylated latex (used in a comparison hereinbelow) having a T.sub.g of about -30.degree. C., is acceptably non-blocking at 49.degree. C. (120.degree. F.), under 689 kPa (100 psi) pressure for 15 min. It was one of the main goals of this invention to provide this `non-blocking` property in a nonwoven bonded with a non-self-supporting film of an acrylate polymer having a T.sub.g lower than -20.degree. C.
An excess of the latex may be used to saturate or super-saturate the nonwoven, in the sense that, besides wetting all the fibers, there is enough aqueous latex held in the pores of the nonwoven, so that suction by vacuum of the super-wet nonwoven, or squeezing between closely spaced rollers, will discharge the excess latex. Upon curing such a super-wet nonwoven, it typically will "breathe" unless it is a relatively "closed" structure of assembled fibers. However, despite there being much more polymer deposited within the non-woven fabric than is required to bond contiguous fibers, curing the latex still forms a network of polymer particles (referred to only as "network" for convenience) sufficient only to coat the fibers, and to bond contiguous portions of fibers which are maintained discrete and separate. It is this "network" which forms the non-self-supporting film less than 0.2 mil thick, supported by the fibers of the nonwoven.
In one typical embodiment, the bonded nonwoven, coated and bonded with the "network", is derived by saturating, impregnating, or otherwise coating a loosely assembled web of fibers with an aqueous itaconic acid-containing (more correctly, methylenebutanedioic acid, or methylenesuccinic acid, and "IA" for brevity), acrylate latex, allowing it to coat and envelop locations where fibers cross or overlap. After the latex is cured, fibers throughout the nonwoven fabric are bonded, one to another, at points where they cross or overlap. If only enough latex is applied to the surface, so that fibers near the surface only, are coated with latex, then upon curing the latex, fibers near the coated surface are bonded at points where they cross or overlap one and another. In either case, because the fibers are separate and discrete, the film supported on the fibers is discontinuous or non-continuous.
In another typical embodiment, a dense, flexible, laminar, nonwoven substrate, for example paper, is coated with a non-self-supporting continuous film of polymer derived by curing a thickened, continuous coating of latex. The paper substrate is a nonwoven fibrous material.
A web of fibers to be bonded may be produced by any one of numerous conventional methods. Fibers may be dry laid, wet laid, or spunbonded. Non-wovens are also produced by melt-blowing, batt drawing, stitchbonding, needle punching, carding, spinning, garnetting, hydroentanglement and spun-lacing techniques. Details relating to manufacture of nonwovens are disclosed in references such as The Nonwovens Handbook edited by Lichstein, B. M. published by INDA Association of the Nonwoven Fabrics Industry (1988); in an article titled "Non-woven Products and Processes" by Philip Smith in Textile Horizons vol 8, No. 4, pg 27-36, Apr. '88; and in the chapter on Nonwoven Fabrics in "Encyclopedia of Polymer Science and Engineering", Second Ed., Wiley-Interscience Publications, John Wiley & Sons 1986; inter alia.
The composition of fibers to be bonded, their length and diameter, and the desired resilience, crush resistance, softness and "hand" are chosen with the end use of the bonded nonwoven product in mind. For example, cotton or cellulose fibers useful in paper-like products are in the range from about 1 mm to about 10 mm long. In textile applications fibers are in the range from about 10 mm to about 75 mm long, and even an essentially continuous fiber may be used. Fibers may be of polyesters such as Dacron, cellulosics such as rayon, polyamides such as nylon, aramids such as Kevlar, and natural fibers such as manila, cotton, and wool.
The resilience and crush resistance of relatively non-resilient and crushable entangled or thermally bonded nonwovens is enhanced by the process of this invention.
The composition used to bond nonwovens comprises an acrylate latex prepared with a major amount of acrylate monomers, no more than about 20 phr (parts by weight based on 100 parts of monomers in the latex) of a monoolefinically unsaturated dicarboxylic acid (MUDA) at least 0.5 phr of which is IA, and a small amount, from about 0.1 phr to about 20 phr of a crosslinking agent, the acrylate monomer being copolymerizable with the crosslinking agent and MUDA. To tailor the properties of the acrylate latex, it may also include at least one monoolefinically unsaturated monomer which is neither a MUDA nor an acrylate. Such a monoolefinically unsaturated monomer, neither a MUDA nor an acrylate, is referred to hereinafter as a non-acrylate monomer. The latex is made by an emulsion polymerization process in which the MUDA, and particularly IA, is initially charged, hereafter "batched", in a conventional semibatch process (hence, "batched-IA" process).
In a semibatch process, commonly used in commercial manufacturing, monomers are proportioned to the reactor. Such proportioning permits control of the reaction, and the properties of the resulting latex. Such processes are versatile and permit production of a wide variety of latex products in a single reactor. Conventionally, the monomers are metered into a mixing tank, along with metered amounts of demineralized water (DW), soaps, deflocculants, etc. and thoroughly mixed to form a "premix". The premix is then flowed into a reactor, about the same size as the mixing tank, but equipped with a cooling jacket and various controls to make sure the reaction proceeds safely according to plan. While the premix is about to be metered into the reactor, and during the reaction, sufficient initiator and activator, along with additional DW, soap, deflocculants and/or buffers are added to the reactor at chosen intervals and predetermined rates found to produce a latex with desired properties.
Whether, a semibatch or batch addition process is used, a conventional process includes a pre-emulsification tank in which all monomers and chosen levels of other ingredients except a water solution of initiator or, in the case of redox initiation, of one initiator component, are emulsified before charging (see chapter on Emulsion Polymerization, Vol 6, pgs 10 et seq. in the "Encyclopedia of Polymer Science and Engineering" supra).
In a conventional process to form a nonwoven with bonding by an acrylate latex, all the unsaturated carboxylic acid, whether mono- or di-, forms a part of the premix. Such a process is disclosed in the aforesaid '665 patent to Kelley. To produce the particular latex I require to bond a nonwoven fabric, at least an equal amount, preferably a major amount, and most preferably, all of the MUDA is placed in the reactor; all the other monomer ingredients being added semi-continuously throughout the reaction at chosen rates.
More specifically, this invention relates to a non-woven coated with an acrylate latex having polymer particles constructed with crosslinked polymer chains in which IA provides them with a unique architecture. Upon being cured, the latex particles coalesce to form the "network" also referred to as a non-self-supporting film supported on the fibers. The nonwoven produced has unique properties derived from the latex made with from about 1 phr to about 10 phr of a particular MUDA, namely IA, optionally in combination with another MUDA present in an amount in the same range.
The IA-containing latex has properties quite different from a latex produced with only the another MUDA, the other ingredients of the latex being the same, provided my novel and unconventional procedure for making the IA-containing latex is used. As will be demonstrated and described herebelow, use of IA produces the latex in which the polymer particles have a unique distribution of carboxyl (COOH) groups, and a characteristic morphology.
Since a lower alkyl acrylate having up to 10 carbon atoms, for example n-butyl acrylate (nBA), and IA have widely differing polarities and water solubilities, the acrylate forms a separate `oil` phase while the IA is present mostly in the aqueous phase. This physical reality, combined with the known retardation effect of IA in such polymerizations, produced undesirable properties in the polymers generally obtained, accounting in no small measure, for the use of a MUDA, in the prior art, in conjunction with a MUMA, the latter being present in a greater concentration than the former. With particular respect to IA, it is known that the homopolymerization is difficult, requiring unusual reaction conditions.
Latexes of acrylates having low T.sub.g, such as of nBA, are known to produce commercial, pressure sensitive adhesives. Hence the resulting non-adhesive outer surfaces of a nonwoven coated with the cured latex made by the "batched-IA" process, was surprising. The latexes I made with conventional "premixed-IA" process variations, produced an excessive amount of coagulum, and when filtered, yielded latexes with poor stability evidenced by a short shelf life. Moreover, the levels of residual monomers, determined gravimetrically, were unacceptably high. In one of the last attempts, I placed all the IA in the reactor and separately dripped in, both a premix in which I had combined the acrylate monomer, the crosslinking agent, and soap in DW, and an initiator drip in which I combined the initiator, some more soap and a buffer such as ammonium carbonate.
Details of this "batched-IA" process, and of the self-supporting, non-porous, continuous film I produced, are set forth in my copending patent application Ser. No. 310,262, filed Feb. 13, 1989, the disclosure of which is incorporated by reference thereto as if fully set forth herein. The film was uniquely characterized by a remarkable combination of elasticity and tensile strength I refer to as "snap" because it is reminiscent of the type of "snap" associated with a common rubber band. To my knowledge, no film of any MUDA-containing acrylate latex in the prior art has "snap". I attribute this unique "snap", quantified herebelow, to the low reactivity of IA, and the unique morphological characteristics of the latex particles in which the COOH groups are distributed in a unique manner.
I attribute the surprising properties of a non-woven coated with the "network" derived from the latex I produced, to the distribution and concentration of COOH groups on particles of the latex which are formed by the "batched-IA" process; to the disposition of the COOH groups in IA relative to the double bond; and, to the relatively difficultly accessible H atoms on the methylene group connected to the COOH group. The structure of IA is written as follows: ##STR2## In addition to having the methylene group between a COOH group and the alpha carbon carrying the double bond, note that both COOH groups are on the same side of that double bond, one of the COOH groups being directly connected to the alpha carbon atom. No other MUDA, and specifically, neither maleic acid, fumaric acid, or citraconic acid has this unique feature.
Because the latex I produced was a carboxylated acrylate latex related to commercially available latexes, I made a comparison and was surprised to find how favorably a film made from my latex compared with one made from a commercial latex. Among such latexes in particular, are Hycar.RTM. 2671 and Hycar.RTM. 26083 brands manufactured by The BFGoodrich Company, the assignee of this application, and one designated TR934, which has been made for many years by Rohm and Haas Company. The Hycar brand latexes contain no MUDA. Because it is not known whether TR934 contains any IA or any other MUDA, an effort was made to determine the presence of such MUDA by solid state NMR (nuclear magnetic resonance) spectroscopy and HPLC (high pressure liquid chromatography), inter alia, but no analytical procedure was capable of identifying what might be a low level, namely less than 10 phr of IA, in the latex.
Searching through patent references assigned to Rohm and Haas Company, and relating to IA-containing carboxylate latexes, I found the disclosure of such a polymer modified with a polyalkylene glycol in the aforesaid '665 patent to Kelley. A specific example of such a polyalkylene glycol-modified polymer was a copolymer of 48% by wt of butyl acrylate, 48% by wt of ethyl acrylate, about 0.5 to 2% of IA and about 2 to 3.5% by wt of N-methylolacrylamide (NMA) (see col 3, lines 20-25). In addition to admonishing that the copolymer in the aqueous dispersion must be obtained by emulsion copolymerization by a mixture of the designated copolymerizable molecules, he taught that "omission of any one of the groups of copolymerizable molecules or substitution for any one of the groups will produce a copolymer which is not completely satisfactory . . . " (see col 3, lines 34-43).
Because of the different reactivities of a MUDA and a MUMA, it is obvious that one would not expect to be able to substitute one for the other and produce a polymer having comparably close properties. Nor would one skilled in the latex art expect to be able, reasonably to predict, how changes in the method of making a latex, using either a MUDA or a MUMA, might affect the particle morphology and the properties of the film made from the latex. One does know that the performance properties of a latex largely depend upon the way it is made (see "Encyclopedia of Chemical Technology", Kirk & Othmer, 3rd ed., Vol 14, pg 92).
It is difficult to predict the properties of any new latex, and more so for a crosslinked carboxylate latex, made with a MUDA present in a greater amount than a MUMA, even if the latex was made with a conventional polymerization process. Having made the latex by an unconventional emulsion polymerization process, I was unprepared for the discovery that a "network" made from the latex would simulate natural rubber (NR), but would imbue a nonwoven fabric with properties which a natural rubber latex cannot.